Antenna for plasma processor apparatus

ABSTRACT

An antenna includes excitation terminals responsive to an RF source to supply an RF electromagnetic field to a plasma that processes a workpiece in a vacuum chamber. The coil includes a transformer having a primary winding coupled to the excitation terminals and a multi-turn plasma excitation secondary winding connected in series with a capacitor.

RELATION TO CO-PENDING APPLICATIONS

Certain aspects of the present application include subject matterdisclosed in the co-pending, commonly assigned Howald et al. applicationSer. Nos. 10/227,275 filed Aug. 26, 2002, (U.S. Pat. No. 6,646,385,which was filed as a continuation of U.S. Pat. No. 6,441,555) and10/200,833 filed Jul. 22, 2002. This application is a divisional of10/334,063, filed Dec. 31, 2002.

FIELD OF INVENTION

The present invention relates generally to plasma processor antennasand, more particularly, to a plasma processor antenna having primary andsecondary windings.

BACKGROUND ART

A typical prior art workpiece processor, as illustrated in FIG. 1,includes vacuum plasma processing chamber assembly 10, a first circuit12 for driving a planar excitation antenna 48 consisting of a coil forexciting ionizable gas in chamber assembly to a plasma state, a secondcircuit 14 for applying RF bias to a workpiece holder in chamberassembly 10, and a controller arrangement 16 responsive to sensors forvarious parameters associated with chamber assembly 10 for derivingcontrol signals for devices affecting the plasma in chamber assembly 10.Controller 16 includes microprocessor 20 which responds to varioussensors associated with chamber assembly 10, as well as circuits 12 and14, and signals from operator input 22, which can be in the form, forexample, of a keyboard. Microprocessor 20 is coupled with memory system24 including hard disk 26, random access memory (RAM) 28 and read onlymemory (ROM) 30. Microprocessor 20 responds to the various signalssupplied to it to drive display 32, which can be a typical computermonitor.

Hard disk 26 and ROM 30 store programs for controlling the operation ofmicroprocessor 20 and preset data associated with different recipes forthe processes performed in chamber assembly 10. The different recipesconcern gas species and flow rates applied to chamber assembly 10 duringdifferent processes, the output power of AC sources included in circuits12 and 14, the vacuum applied to the interior of chamber assembly 10,and initial values of variable reactances included in matching networksof circuits 12 and 14.

Plasma chamber assembly 10 includes chamber 40 having non-magneticcylindrical side wall 42 and non-magnetic base 44, both of which arefrequently metal and electrically grounded. Dielectric, typicallyquartz, window 46 is fixedly positioned on the top edge of wall 42.

Wall 42, base 44 and window 46 are rigidly connected to each other bysuitable gaskets to enable a vacuum to be established within theinterior of chamber 40. Plasma excitation antenna 48 includes coil 49,that is planar or dome shaped, and can be configured as disclosed inOgle, U.S. Pat. No. 4,948,458 or Holland et al., U.S. Pat. No. 5,759,280or Holland et al, U.S. Pat. No. 5,800,619 sits on or in very closeproximity to the upper face of window 46. Antenna 48 reactively suppliesmagnetic and electric RF fields to the interior of chamber 40, to exciteionizable gas in the chamber to a plasma, schematically illustrated inFIG. 1 by reference numeral 50.

The upper face of base 44 carries holder (i.e. chuck) 52 for workpiece54, which is typically a circular semiconductor wafer, a rectangulardielectric plate such as used in flat panel displays or a metal plate.Workpiece holder 52 typically includes metal plate electrode 56 whichcarries dielectric layer 58 and sits on dielectric layer 60, which iscarried by the upper face of base 44. A workpiece handling mechanism(not shown) places workpiece 54 on the upper face of dielectric layer58. Workpiece 54 is cooled by supplying helium from a suitable source 62to the underside of dielectric layer 58 via conduit 64 and grooves (notshown) in electrode 56. With workpiece 54 in place on dielectric layer58, d.c. source 66 supplies a suitable voltage through a switch (notshown) to electrode 56 to clamp, i.e., chuck, workpiece 54 to holder 52.

With workpiece 54 secured in place on chuck 52, one or more ionizablegases from one or more sources 68 flow into the interior of chamber 40through conduit 70 and port 72 in sidewall 42. For convenience, only onegas source 68 is shown in FIG. 1. The interior of conduit 70 includesvalve 74 and flow rate gauge 76 for respectively controlling the flowrate of gas flowing through port 72 into chamber 40 and measuring thegas flow rate through port 72. Valve 74 responds to a signalmicroprocessor 20 derives, while gauge 76 supplies the microprocessorwith an electric signal indicative of the gas flow rate in conduit 70.Memory system 24 stores for each recipe of each workpiece 54 processedin chamber 40 a signal indicative of desired gas flow rate in conduit70. Microprocessor 20 responds to the signal memory system 24 stores fordesired flow rate and the monitored flow rate signal gauge 76 derives tocontrol valve 74 accordingly.

Vacuum pump 80, connected to port 82 in base 44 of chamber 40 by conduit84, evacuates the interior of the chamber to a suitable pressure,typically in the range of one to one hundred milliTorr. Pressure gauge86, in the interior of chamber 40, supplies microprocessor 20 with asignal indicative of the vacuum pressure in chamber 40.

Memory system 24 stores for each recipe a signal indicative of desiredvacuum pressure for the interior of chamber 40. Microprocessor 20responds to the stored desired pressure signal memory system 24 derivesfor each recipe and an electric signal from pressure gauge 86 to supplyan electric signal to vacuum pump 80 to maintain the pressure in chamber40 at the set point or predetermined value for each recipe.

Optical spectrometer 90 monitors the optical emission of plasma 50 byresponding to optical energy emitted by the plasma and coupled to thespectrometer via window 92 in side wall 42. Spectrometer 90 responds tothe optical energy emitted by plasma 50 to supply an electric signal tomicroprocessor 20. Microprocessor 20 responds to the signal spectrometer90 derives to detect an end point of the process (either etching ordeposition) that plasma 50 is performing on workpiece 54. Microprocessor20 responds to the signal spectrometer 90 derives and a signal memorysystem 24 stores indicative of a characteristic of the output of thespectrometer associated with an end point to supply the memory with anappropriate signal to indicate the recipe has been completed.Microprocessor 20 then responds to signals from memory system 24 to stopcertain activities associated with the completed recipe and initiate anew recipe on the workpiece previously processed in chamber 40 orcommands release of workpiece 54 from chuck 52 and transfer of a newworkpiece to the chuck, followed by instigation of another series ofprocessing recipes.

Excitation circuit 12 for driving coil 49 of antenna 48 includesconstant or variable frequency RF source 100 (see Barnes et al U.S. Pat.No. 5,892,198), typically having a frequency of 4.0±10% MHz or 13.56±10%MHz. Source 100 drives variable gain power amplifier 102, typicallyhaving an output power in the range between 100 and 3000 watts.Amplifier 102 typically has a 50 ohm output impedance all of which isresistive and none of which is reactive. Hence, the impedance seenlooking back into the output terminals of amplifier 102 is typicallyrepresented by (50+j0) ohms, and cable 106 is chosen to have acharacteristic impedance of 50 ohms.

For any particular recipe, memory system 24 stores a signal for desiredoutput power of amplifier 112. Memory system 24 supplies the desiredoutput power of amplifier 102 to the amplifier by way of microprocessor20. The output power of amplifier 102 can be controlled in an open loopmanner in response to the signals stored in memory system 24 or controlof the output power of amplifier 102 can be on a closed loop feedbackbasis, as known in the art.

The output power of amplifier 102 drives coil 49 via cable 106 andmatching network 108. Matching network 108, configured as a “T,”includes two series legs including variable capacitors 112 and 116, aswell as a shunt leg including fixed capacitor 114. The antenna 48includes excitation terminals 122 and 124, respectively connected to (1)a first end of coil 49 and one electrode of capacitor 112 and (2) asecond end of coil 49 and a first electrode of series capacitor 126,having a grounded second electrode; or terminal 124 can be connecteddirectly to ground. The value of capacitor 126 is preferably selected asdescribed in the commonly assigned, previously mentioned, Holland et al.'200 patent.

Electric motors 118 and 120, preferably of the step type, respond tosignals from microprocessor 20 to control the values of capacitors 112and 116 in relatively small increments to maintain an impedance matchbetween the impedance seen by looking from the output terminals ofamplifier 102 into cable 106 and by looking from cable 106 into theoutput terminals of amplifier 102. Hence, for the previously described(50+j0) ohm output impedance of amplifier 102 and 50 ohm characteristicimpedance of cable 106, microprocessor 20 controls motors 118 and 120 sothe impedance seen looking from cable 106 into matching network 108 isas close as possible to a matched impedance of (50+j0) ohms.Alternatively, microprocessor 20 controls the frequency of source 100and the capacitance of capacitor 116 to achieve a matched impedancebetween the source and the load it drives. As a result of a matchedimpedance being attained, the current flowing through capacitors 112 and126 and the leads connecting the capacitors to terminals 122 and 124, istypically within a couple of percent of its very high maximum value. Thevery high current in these leads has an adverse effect on the uniformityof the density of plasma 50.

To control motors 118 and 120 or the frequency of source 100 and motor120 to maintain matched conditions between the impedance seen lookinginto the output terminals of amplifier 102 and the impedance amplifier102 drives, microprocessor 20 responds to signals from conventionalsensor arrangement 104. The signals are indicative of the impedance seenlooking from cable 106 into matching network 108; usually the signalsrepresent the absolute values of the current and voltage reflectedtoward the sensor from capacitor 118, and the phase angle between thereflected current and voltage. Alternatively, sensors are provided forderiving signals indicative of the power that amplifier 102 supplies toits output terminals and the power reflected by cable 106 back to theoutput of amplifier 102. Microprocessor 20 responds, in one of severalknown manners, to the sensed signals sensor arrangement 104 derives tocontrol motors 118 and 120 or the frequency of source 100 and motor 120to attain the matched condition.

Because of variations in conditions in the interior of chamber 40 whichaffect plasma 50, the plasma has a variable impedance. The conditionsare aberrations in the flow rate and species of the gas flowing throughport 72, aberrations in the pressure in chamber 40 and other factors. Inaddition, noise is sometimes supplied to motors 118 and 120 causing themotors to change the values of capacitors 112 and 116. All of thesefactors affect the impedance reflected by the load including plasma 50back to the output terminals of amplifier 102. Microprocessor 20responds to the output signals of sensor 104, to vary the values ofcapacitors 112 and 116 or the frequency of source 100, to maintain theimpedance driven by the output terminals of amplifier 102 matched to theoutput impedance of the amplifier.

Circuit 14 for supplying RF bias to workpiece 54 via electrode 56 has aconstruction somewhat similar to circuit 12. Circuit 14 includesconstant frequency RF source 130, typically having a frequency such as400 kHz, 2.0 MHz or 13.56 MHz. The constant frequency output of source130 drives variable gain power amplifier 132, which in turn drives acascaded arrangement including directional coupler 134, cable 136 andmatching network 138. Matching network 138 includes a series legcomprising the series combination of fixed inductor 140 and variablecapacitor 142, as well as a shunt leg including fixed inductor 144 andvariable capacitor 146. Motors 148 and 150, which are preferably stepmotors, vary the values of capacitors 142 and 146, respectively, inresponse to signals from microprocessor 20.

Output terminal 152 of matching network 138 supplies an RF bias voltageto electrode 56 by way of series coupling capacitor 154 which isolatesmatching network 138 from the chucking voltage of d.c. source 66. The RFenergy circuit 14 applies to electrode 56 is capacitively coupled viadielectric layer 48, workpiece 54 and a plasma sheath between theworkpiece and plasma to a portion of plasma 50 in close proximity withchuck 52. The RF energy chuck 52 couples to plasma 50 establishes a d.c.bias in the plasma; the d.c. bias typically has values between 50 and1000 volts. The d.c. bias resulting from the RF energy circuit 14applies to chuck 52 accelerates ions in the plasma 50 to workpiece 54.

Microprocessor 20 responds to signals indicative of the impedance seenlooking from cable 136 into matching network 138, as derived by a knownsensor arrangement 139, to control motors 148 and 150 and the values ofcapacitors 142 and 146 in a manner similar to that described supra withregard to control of capacitors 112 and 116 of matching network 108.

For each process recipe, memory system 24 stores a set point signal forthe net power flowing from directional coupler 134 into cable 136. Thenet power flowing from directional coupler 134 into cable 136 equals theoutput power of amplifier 132 minus the power reflected from the loadand matching network 138 back through cable 136 to the terminals ofdirectional coupler 134 connected to cable 136. Memory system 28supplies the net power set point signal associated with circuit 14 tomicroprocessor 20. Microprocessor 20 responds to the net power set pointsignal associated with circuit 14 and the output signals thatdirectional coupler 134 supply to power sensor arrangement 141. Powersensor arrangement 141 derives signals indicative of output power ofamplifier 132 and power reflected by cable 136 back toward the outputterminals of amplifier 132.

FIG. 2 is a perspective view of an antenna consisting of a planar coilof the type schematically illustrated in FIG. 6 of the previouslymentioned '619 patent and which has been incorporated as the coil ofantenna 48 in processors of the type illustrated in FIG. 1. The coilillustrated in FIG. 2 includes a single winding 160 including inner andouter concentric metal turns 162 and 164, each of which has a squarecross-section and is shaped as a sector of a circle extending through anangle of approximately 340 degrees. Opposite ends of turns 162 and 164respectively include excitation terminals 166 and 168, respectivelyconnected by metal posts (i.e. current feeds) 170 and 172 to oneelectrode of capacitor 112 of matching network 108 and to one electrodeof capacitor 126; alternatively, post 172 connects excitation terminal168 to ground directly. Consequently, the RF (i.e. AC) current whichflows in posts 170 and 172 is approximately equal to the RF currentwhich flows in turns 162 and 164. The ends of turns 162 and 164 remotefrom terminals 166 and 168 are connected to each other by straight metalstrut 174 that extends generally radially between turns 162 and 164 andhas the same cross-sectional configuration as the turns.

The two turn coil of FIG. 2 differs from an ideal two turn coil whichconsists of two coaxial circular loops having constant, equal amplitudeRF currents flowing therein throughout the length of each loop. Such anideal two turn coil would provide, to the plasma 50 in chamber 40,electric and magnetic fields having complete cylindrical symmetry. Thecoil of FIG. 2, as well as all practical coils that can be used as thecoil of antenna 48, has connections (such as strut 174 that connectsturns 162 and 164) between any loops or windings included in the coil,and current feed points, such as excitation terminals 166 and 168 thatconnect posts 170 and 172 to turns 162 and 164. These connectionsprevent all practical coils from having the complete cylindricalsymmetry of the idealized coil.

The currents in the practical coil of FIG. 2 can be expressed as the sumof the current in the ideal portions of the coil, i.e., turns 162 and164, plus the current in a hypothetical perturbation coil that includesterminals 166 and 168, posts 170 and 172, and strut 174. Thehypothetical perturbation coil thus includes the effects of the currentfeeds formed by posts 170 and 172, the “missing” sections of the loopsformed by turns 162 and 164, as well as strut 174 which forms aconnection between the loops formed by turns 162 and 164. The currentflowing in the hypothetical perturbation coil, including the highcurrent flowing in the current feeds formed by posts 170 and 172, has atendency to cause azimuthal asymmetry in the magnetic field coupled bythe coil to the plasma, resulting in azimuthal asymmetry in the plasmadensity processing the workpiece.

One object of the present invention is to provide a new and improvedplasma processor including a plasma having a density with reducedazimuthal asymmetry.

Another object of the present invention is to provide a new and improvedantenna arrangement for a plasma processor.

An added object is to provide a new and improved plasma processorantenna arrangement for enabling the plasma of the processor to havedensity with relatively low asymmetry.

An additional object of the present invention is to provide a new andimproved antenna arrangement for a plasma processor, wherein the antennaarrangement is arranged so that the perturbing effects of RF feeds thatsupply current to the antenna arrangement are reduced compared to atypical prior art arrangement.

A further object of the present invention is to provide a new andimproved plasma processor wherein a relatively high current flows in aplasma excitation coil of an antenna while a substantially loweramplitude current flows in the leads connecting the antenna to circuitrywhich drives the antenna.

SUMMARY OF THE INVENTION

One aspect of the invention relates to a plasma processor antennaadapted to be driven by power from an AC source for exciting a plasma.The antenna comprises first and second excitation terminals, as well asa coil including a primary winding having opposite ends respectivelycoupled with the first and second excitation terminals, and a secondarywinding reactively coupled with the primary winding. A capacitor hasfirst and second opposite electrodes respectively connected in serieswith the secondary winding.

A further aspect of the invention relates to an antenna for a plasmaprocessor. The antenna is adapted to be driven by power from an ACsource for exciting a plasma and comprises first and second excitationterminals, as well as a coil including (1) a primary winding havingopposite ends respectively coupled with the first and second excitationterminals, and (2) a secondary winding reactively coupled with theprimary winding. The secondary winding includes plural turns.

Preferably, the turns are in different parallel planes adapted to bespatially parallel to a coupling window of a vacuum chamber of theprocessor. The plural turns are also preferably concentric with an axisof the coil. In such case, the primary winding includes at least onefurther turn that is concentric with the coil axis and is in a planespatially parallel to the plural turns of the secondary winding and thesecondary winding includes multiple turns in each of the planes. Theturns of the secondary winding are preferably connected in series witheach other and arranged so AC current induced to them in response toexcitation of the primary winding flows in the same direction throughhalf planes extending from the axis through the turns.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed descriptions of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1, as previously described, is a schematic diagram of a prior artvacuum plasma processor;

FIG. 2, as previously described, is a perspective view of an antennacoil of the type which has been employed in the vacuum plasma processorof FIG. 1;

FIGS. 3-12 are schematic diagrams of different embodiments of drivenetworks in combination with several antenna embodiments, in accordancewith preferred embodiments of the present invention;

FIG. 13 is a perspective view of the antenna included in the embodimentsof FIGS. 3, 4 and 8;

FIG. 14 is a perspective view of a the antenna included in theembodiments of FIGS. 5 and 7;

FIG. 15 is a perspective view of an antenna including a transformerhaving primary and secondary windings, as schematically illustrated inFIGS. 10 and 11; and

FIG. 16 is a schematic perspective view of an antenna including atransformer having a primary winding and a four-turn secondary winding,as schematically illustrated in FIG. 12.

DETAILED DESCRIPTION of FIGS. 3-16

Reference is now made to the schematic diagram of FIG. 3 wherein fixedfrequency RF source 232 (typically having a frequency of 4.0 MHz or13.56 MHz) is illustrated as having an output that drives cable 106,having an output connected, via sensor 104, to one electrode ofvariable, series connected capacitor 212 of matching network 211. Asecond electrode of capacitor 212 is connected by current feed or post213 to excitation terminal 214 of antenna 215, including coil 216;details of antenna 215 are illustrated in FIG. 13. Antenna 215 replacesantenna 48, FIG. 1. A first terminal of coil 216 is connected togrounded excitation terminal 220 via the series connection of ammeter228 and fixed capacitor 426 (which is the equivalent of capacitor 126,FIG. 1).

The impedance of plasma 50 is indicated in FIG. 3 by the seriesimpedance Z_(P) (box 218) between the first end of coil 216 andcapacitor 426. Variable capacitor 223, preferably a semiconductor of thetype that is electronically controlled by a voltage applied to a controlelectrode thereof, is connected in series with excitation terminal 214and a second end of coil 216. Opposite electrodes of fixed capacitor 224are respectively connected to excitation terminals 214 and 220.Capacitors 212, 223 and 224 form impedance matching network 211 similarto impedance matching network 108 of FIG. 1. However, capacitors 223 and224 are relocated onto antenna 215 so that the relatively high currentthat flows in these capacitors does not flow through rf feeds 213 and222. In contrast, capacitors 112 and 114 of matching network 108 of theprior art are physically removed from antenna 48 and the relatively highcurrents that flow through capacitors 112 and 114 also flow through therf leads that connect capacitors 112 and 114 to antenna 48.

A controller including microprocessor 229 controls the values ofcapacitors 212 and 223 to achieve (1) maximum current in ammeter 228and, equivalently, (2) impedance matching between the output of source210 and the load it drives, at the input of cable 106. To achieve theseresults, microprocessor 229 responds to the output of ammeter 228 or ofsensor 104 to adjust (1) the capacitances of capacitors 212 and 223until the output impedance of source 232 and the impedance the sourcedrives are matched, or (2) until the current in coil 216, as measured byammeter 228, is maximized. In response to one of these criteria beingachieved, usually in an iterative manner, the current in the feeds islower (typically about one-half to one-fifth) than the current in branch225, and problems associated with high current flowing in feeds 213 and222 discussed previously are avoided. Some of the current in coil 216also flows in capacitor 224 and some in feeds 213 and 222. The phases ofthe instantaneous currents flowing in branch 225, and feeds 213 and 222differ, such that the phase of the instantaneous current in coil 216 isabout 90° from the instantaneous currents in feeds 213 and 222.Microprocessor 229 responds to indications of the current magnitudesensed by ammeter 228 or the phase angle, voltage magnitude and currentmagnitude as indicated by output signals of sensor 104. Microprocessor229 responds to the output signals of ammeter 228 or sensor 104 tocontrol the capacitances of capacitors 212 and 223 to maximize thecurrent in coil 216 or so the impedance seen looking from cable 106 intosensor 104 equals the characteristic impedance of cable 106, i.e., amatch is attained. If necessary, microprocessor 229 responds to theoutputs of sensor 104 and ammeter 228 to iteratively control the valuesof capacitors 212 and 223 so that the current in coil 216 is maximizedand the output impedance of source 210 is matched to the load it drives.Maximizing or nearly maximizing the current in coil 216 causes theelectromagnetic field the coil supplies to the plasma to be maximized.

Reference is now made to FIG. 4 of the drawing, a schematic diagram ofan embodiment which is similar to the embodiment of FIG. 3. FIG. 4 isthe same as FIG. 3 except that ammeter 228 is eliminated andmicroprocessor 230 replaces microprocessor 229. Microprocessor 230includes a conventional, prior art algorithm for controlling theimpedances of a matching network of a plasma processor. Microprocessor230 thus can be the same as microprocessor 20 of FIG. 1.

Microprocessor 230 controls capacitor 223 that is part of antenna 215and part of matching network 211 to achieve an impedance match betweensource 232 and the load it drives. Such an impedance match isaccompanied by the current in coil 216 being greater than the currentflowing in feeds 213 and 222 from circuitry outside antenna 215. Thecoil current is typically two to five times current in feeds 213 and222.

When microprocessor 230 has controlled capacitors 212 and 233 to achieveimpedance matching of source 232 to the impedance it drives, the currentthat flows in coil 216 is maximized. It can be shown that a matchbetween the output impedance of source 232 and the load the sourcedrives is achieved when the current in coil 216 (I_(C)) is related tothe current flowing in feeds 213 and 222 (I_(IN)) from circuitry outsideantenna 215 in accordance with$\frac{I_{c}}{I_{IN}} = \sqrt{\frac{R_{0}}{R_{P}}}$where

-   -   R₀=the characteristic impedance of cable 106, and    -   R_(P)=the real part of impedance Z_(P) of plasma 50 as coupled        to coil 216.        Typical values of R₀ and R_(P) are respectively 50 ohms and 2-10        ohms, resulting in $\begin{matrix}        {\frac{I_{c}}{I_{IN}} = {\sqrt{\frac{50}{2}} = {\sqrt{25} = {5\quad{and}}}}} \\        {\frac{I_{c}}{I_{IN}} = {\sqrt{\frac{50}{10}} = {\sqrt{5} = 2.2}}}        \end{matrix}$        Hence, for the optimum impedance matching and maximum current in        coil 216 (i.e., a resonant condition for the load that source        216 drives) the current in coil 216 is typically about two to        five times the current flowing via feeds 213 and 224 through        excitation terminals 214 and 220. However, for some non-optimum        conditions, e.g., there is a slight impedance mismatch between        source 232 and the load it drives or the current flowing in coil        216 is somewhat less than the maximum current that can flow in        the coil for the frequency of source 232, the current in coil        216 is also about twice the current flowing through excitation        terminals 214 and 220 via feeds 213 and 224, for values of        plasma resistance less than 10 ohms.

It can be shown that matching between the output impedance of source 232and the load the source drives results when $\begin{matrix}{\frac{1}{\omega\quad C_{3}} = {{\omega\quad L} + {\omega\quad L_{P}} - \frac{1}{\omega\quad C_{2}} + {\sqrt{\frac{R_{P}}{R_{0}\omega^{2}C_{2}^{2}} - R_{P}^{2}}\quad{and}}}} \\{\frac{1}{\omega\quad C_{1}} = {{- \frac{1}{\omega\quad C_{2}}} \pm \sqrt{\frac{R_{0}}{R_{P}\omega^{2}C_{2}^{2}} - R_{0}^{2}}}}\end{matrix}$where

-   -   ω=2π×f    -   f=frequency of source 232    -   C₁=capacitance of capacitor 212    -   C₂=capacitance of capacitor 224 and    -   C₃=series capacitance of capacitors 223 and 426    -   L=inductance of coil 216    -   R_(P)=real part of effective plasma impedance Z_(P) (218), and    -   L_(P)=inductance of effective plasma impedance Z_(P) (218)        For typical values of: f=13.56 MHz, R₀=50 ohms, R_(P)=5 ohms,        C₂=100 pf, L=2.2 μH, and L_(P)=−0.2 μH, C₁=47 pf and C₃=131 pf        (by taking the positive value of the square root for the values        of C₁ and C₃).

Reference is now made to FIG. 5, which is the same as FIG. 4 except thatcoil 216 and capacitor 223 are respectively replaced by coil 240 andvariable capacitor 242. The capacitances of capacitors 212 and 242 arecontrolled by a microprocessor (not shown in FIG. 5) in the same waythat microprocessor 230 controls capacitors 212 and 223.

Coil 240 includes two series connected segments 244 and 246, having agap between them. The gap is defined by a pair of terminals 248 and 250of coil 240. Terminals 248 and 250 are respectively connected toopposite electrodes of capacitor 242 so that capacitor 242 is connectedin series with segments 244 and 246. The gap between terminals 248 and250 is preferably located at a location in coil 240 which providesoptimum distribution of current and voltage along the length of thecoil, as described in U.S. Pat. No. 6,441,555.

Reference is now made to FIG. 6, which is the same as FIG. 5, exceptthat variable capacitor 242 is replaced by variable capacitor 602 andfixed capacitor 604. Capacitors 212 and 602 are controlled by amicroprocessor (not shown in FIG. 6) in the same way that microprocessor230 controls capacitors 212 and 223.

Variable capacitor 602 is connected between terminal 214 and one end ofcoil segment 244, the other end of which is connected to one electrodeof capacitor 604. A second electrode of capacitor 604 is connected toone end of coil segment 246. The location of capacitor 604 in coil 240is typically different from the location of capacitor 242 becausecapacitor 602 changes the voltage and current distribution in coil 240.

Reference is now made to FIG. 7 of the drawing, which is the same asFIG. 5 except that variable frequency source 233 replaces fixedfrequency source 232, microprocessor 231 replaces the microprocessor ofFIG. 5 and fixed capacitor 702 replaces variable capacitor 242. Variablefrequency source 233 typically has a range of frequencies that istypically about ±10% of the center frequency of the source.Microprocessor 231 responds to the indications derived by sensor 104, tocontrol (in a known manner) the frequency of source 233 and thecapacitance of capacitor 212 until the microprocessor detects animpedance match between the source and the load it drives. Thecombination of indications sensor 104 derives indicates the impedancematch between source 233 and the load it drives. As a result ofmicroprocessor 231 controlling the frequency of source 233 and thecapacitance of capacitor 212 to achieve the impedance match, plasmaimpedance 218, capacitors 426 and 702, as well as coil segments 244 and246, have impedances that cause the current in coil segments 244 and 246to exceed the current flowing through feeds 213 and 222 from thecircuitry outside antenna 215; typically, the current in coil segments244 and 246 is about two to five times the current in feeds 213 and 222since $\frac{I_{c}}{I_{IN}} = \sqrt{\frac{R_{0}}{R_{P}}}$and the values of R₀ and R_(P) are typically as previously stated. It isto be understood that the frequency of source 233 can alternatively becontrolled in response to a signal that detects a maximum current inbranch 225, as described in connection with FIG. 3.

Reference is now made to FIG. 8 that is the same as FIG. 4 except thatvariable frequency source 233 replaces fixed frequency source 232, amicroprocessor (not shown in FIG. 8) that is the same as microprocessor231 replaces microprocessor 230 and fixed capacitor 802 replacesvariable capacitor 223. The microprocessor of FIG. 8 controls thefrequency of source 233 and the value of capacitor 212 as described inconnection with FIG. 7.

Reference is now made to FIG. 9, which is the same as FIG. 6, exceptthat variable frequency source 233 replaces fixed frequency source 232,a microprocessor (not shown) that is the same as microprocessor 231replaces the microprocessor of FIG. 6, and fixed capacitor 902 replacesvariable capacitor 602. The microprocessor of FIG. 9 controls thefrequency of source 233 and the value of capacitor 212 as described inconnection with FIG. 7.

Reference is now made to FIG. 10 wherein the antenna of FIG. 4 ismodified to include air core transformer 1002 including primary winding1004 and secondary winding 1006 that drives the plasma load impedance218 and is in series with variable capacitor 1008. Opposite ends ofprimary winding 1004 are connected to excitation terminals 214 and 220,so that the primary winding is in series with variable capacitor 212 ofthe matching network. Primary winding 1004 can thus be considered aspart of the matching network. Fixed frequency source 232 is connected inseries with cable 106, variable capacitor 212, RF feeds 213 and 224, andwinding 1004 so that substantially the same current flows in winding1004 as is derived by source 232.

Secondary winding 1006, variable capacitor 1008 and plasma impedance 218are in series in nearly resonant closed loop 1010. A microprocessor (notshown) that is the same as microprocessor 230 responds to the outputsignals of sensor 104 to control the capacitances 212 and 1008. Thecontrol is such that there is an impedance match between source 232 andthe load the source drives. When the match occurs, loop 1010 has animpedance with a resonant frequency that is nearly the same as thefrequency of the source, i.e., load 1010 is nearly resonant to the fixedfrequency of source 232.

To prevent the high amplitude, near resonant current that flows in loop1010 from being coupled to primary winding 1004, as well as feeds 213and 224, windings 1004 and 1006 are loosely coupled. The loose couplingbetween windings 1004 and 1006 results in the impedance of loop 1010that is nearly resonant to the frequency of source 232 being coupledwith a large resistive component to winding 1004. Consequently, thecurrent flowing through feeds 213 and 224 has an amplitude that is muchlower than the current flowing in loop 1010. A typical couplingcoefficient between windings 1004 and 1006 to achieve the desired loosecoupling effect and provide a relatively efficient transfer of powerfrom winding 1004 to winding 1006 is in the range of about 0.1 to 0.3.This coefficient range results in transformer 1002 having an efficiencyof about 70 percent to 90 percent.

Reference is now made to FIG. 11, a modification of FIG. 10, whereinvariable frequency source 233 replaces fixed frequency source 232, amicroprocessor (not shown) configured the same as microprocessor 231replaces the microprocessor of FIG. 10 and fixed capacitor 1102 replacesvariable capacitor 1008. Microprocessor 233 responds to the outputsignals of sensor 104 to control the (1) value of capacitor 212 andfrequency of source 233 to provide an impedance match between source 232and the load it drives. When the match occurs, the frequency of source233 and the resonant frequency of loop 1010 are nearly the same.

Reference is now made to FIG. 12, which is the same as FIG. 11, exceptthat the antenna includes air core transformer 257 including primarywinding 258 and secondary windings 346 and 348, both of which (1) areloosely coupled with the primary winding, and (2) drive the plasmaimpedance 218. As described in connection with FIG. 16, the inductiveand capacitive coupling between windings 346 and 348 and plasmaimpedance 218 is greater than the inductive and capacitive couplingbetween winding 258 and the plasma impedance. Secondary windings 346 and348 are connected in series with fixed capacitors 254 and 256 to formclosed loop 2002. Capacitors 254 and 256 are located to provide adesired distribution of current and voltage along the length of theclosed loop 1202 formed by windings 346 and 348, as well as capacitors254 and 256. The combined coupling coefficient between winding 258 andwindings 346 and 348 is approximately in the 0.1 to 0.3 range. Amicroprocessor (not shown) that is the same as microprocessor 231responds to output signals of sensor 104 to control the value ofcapacitor 212 and the frequency of source 233 to provide an impedancematch between source 232 and the load it drives. When the impedancematch is achieved, the source frequency and the resonant frequency ofloop 1202 are nearly the same.

It is to be understood that an ammeter can be connected to loops 1010and 1202 to control the capacitance of capacitor 1008 (FIG. 10) and thefrequency of source 233 (FIGS. 11 and 12) so maximum current in loops1010 and 1202 is achieved. Maximizing the current in loops 1010 and 1202results from the impedances of the loop being resonant to thefrequencies of sources 232 or 233.

It is also to be understood that it is not necessary for the current inthe plasma excitation coil to be maximized or nearly resonant to thefrequency of the fixed or variable frequency sources of FIGS. 10-12 orfor an exact impedance match to be achieved to enable the current in theexcitation coil to exceed the current flowing in leads 213 and 222 viacircuitry outside the antenna, although these are the most desirableconditions for efficient transfer of electromagnetic energy from thecoil to plasma. The value of the variable capacitor series connected tothe coil or the frequency of the source can be adjusted by using otherarrangements that cause the coil current to exceed the current in the RFfeeds. For example, the source frequency and capacitor(s) seriesconnected to the coil can be fixed at values which cause the coilcurrent to exceed the current in the RF feeds and approximate matchingcan be attained by controlling the value of only capacitor 212.

In the transformers of FIGS. 10-12, the primary and secondary windingsare only reactively coupled to each other and there is no directconnection from the secondary windings 262 to the RF sources. Secondarywinding 1006 is in close proximity to window 46 so that secondarywinding 1006 is the main supplier of magnetic and electric fields toplasma 50, i.e., secondary winding 1006 supplies a considerably greateramount of magnetic and electric fields to the plasma than primarywinding 1004. Windings 1004 and 1006 are typically substantially planar,and lie in planes parallel to each other and a planar face of window 46.

In a preferred embodiment of FIG. 12, primary winding 258 includes asingle loop in a first plane, while secondary windings 346 and 348together include four coaxial turns, which are also coaxial with thesingle loop of primary winding 258 and are closed on themselves. Thecoaxial turns of windings 346 and 348 are arranged so that the two turnsof winding 346 are in a second plane and the two turns of winding 348are in a third plane; the second and third planes are parallel to thefirst plane. The planes of the turns of windings 258, 346 and 348 areparallel to a plane of a face of window 46, such that the planes ofsecondary windings 346 and 348 are closer to window 46 than the plane ofprimary winding 258, with winding 348 being closer to the window thanwinding 346. The coaxial turns of windings 346 and 348 are connected toeach other so that current instantaneously flows in the same directionin the turns of windings 346 and 348 in a half plane intersecting acommon axis of the loop of primary winding 258 and the turns of windings346 and 348.

Reference is now made to FIG. 13 of the drawing, a perspective view ofthe antenna of each of FIGS. 3, 4 and 8, in combination with currentfeeds or posts 213 and 222. (To simplify the drawing of FIG. 13, ammeter228 of FIG. 3 is omitted.) Antenna coil 216 includes a single planar,substantially circular nonmagnetic metal loop 310, formed as a sector ofa circle, wherein the sector has an extent of approximately 340 degrees.Loop 310 is disposed in a plane parallel and in close proximity to theupper face of window 46. Loop 310 has a vertically extending centralaxis that is coincident with the center of workpiece 54, when theworkpiece is properly positioned on chuck 52.

Loop 310 includes a pair of end terminals 312 and 314 electricallycoupled with excitation terminals 214 and 220. Excitation terminals 214and 220 are at the ends of and are ohmically connected to verticallyextending current feeds or posts 213 and 222, that are directed upwardlyfrom the plane of loop 310. Capacitor 224 bridges a gap betweenexcitation terminals 214 and 220, while capacitor 223 bridges a gap inloop 310 between excitation terminals 214 and end terminal 312.Excitation terminal 220 and end terminal 314 are coincident.

Reference is now made to FIG. 14 of the drawing, a perspective view ofthe antenna of FIGS. 5-7, in combination with current feeds or posts 213and 222. The antenna of FIG. 14 includes coil 240 having a singleplanar, non-magnetic, metal, substantially circular loop 316, formed astwo sectors of a circle, wherein the sectors have a combined extent ofapproximately 340 degrees. The sectors form segments 244 and 246. Loop316 is disposed in a plane parallel and in close proximity to the upperface of window 46. Loop 316 has a vertically extending axis that iscoincident with the center of workpiece 54, when the workpiece isproperly positioned on chuck 52.

Loop 316 includes a pair of end terminals 318 and 320 that arerespectively coincident with excitation terminals 214 and 220. Capacitor234 bridges the gap between excitation terminals 214 and 220, whilecapacitor 242 bridges a gap between terminals 248 and 250 of loop 316.Each of the gaps has an arcuate extend of about 10 degrees. While thegap in loop 316 between terminals 248 and 250 is illustrated in FIG. 14as diametrically opposed to the gap between excitation terminals 214 and220, it is to be understood that the gap in loop 316 is preferably at alocation in the loop which results in optimum distribution of currentand voltage in the loop. Also, it is be understood that loops 310 and316 can have plural capacitors between the end terminals thereof. Forexample, the embodiments of FIGS. 6 and 9 are realized by a combinationof FIGS. 13 and 14.

Reference is now made to FIG. 15 of the drawing, a perspective view ofthe antenna illustrated in FIGS. 10 and 11. The antenna of FIG. 8,illustrated in FIG. 15 in combination with current feeds 213 and 222,includes primary winding 1004 and secondary winding 1006. Secondarywinding 1006 is only reactively coupled to primary winding 1004, suchthat there is no ohmic connection from source 232 or 233 or the primarywinding to the secondary winding. Primary winding 1004 includes a singleplanar, non-magnetic, metal substantially circular loop 1502, formed asa sector of a circle, wherein the sector has an extent of approximately350 degrees. Primary winding 1004 has a pair of opposite ends coincidentwith and ohmically connected to excitation terminals 276 and 278, whichare respectively connected to upwardly extending current feeds 213 and222. Secondary winding 1006 includes a single planar, non-magnetic,metal, circular loop 1504 including a gap between a pair of terminals1506 and 1508. Capacitor 1008 (FIG. 10) or 1102 (FIG. 11) is located inthe gap such that opposite electrodes thereof are respectively connectedto terminals 1506 and 1508. Again, it is be understood that loop 326 caninclude plural gaps, each including a separate capacitor.

Loops 1502 and 1504, as well as the upper face of window 46, are inmutually parallel planes such that loop 1504 is in close proximity towindow 46 and loop 1506 is remote from the window. The distanceseparating loop 1506 and window 46 is sufficient to provide substantialdecoupling of the magnetic and electric fields originating in loop 1506from plasma 50 in vacuum chamber 40. The distance between loop 1506 andwindow 46 is such that there is substantial coupling of the magnetic andelectric fields originating in loop 1506 to plasma 50. The distancebetween loops 1504 and 1506 is such that there is a transformer couplingcoefficient between 0.1 and 0.3 between these loops. Typically, loop1506 has a greater diameter than loop 1504, to assist in providingproper coupling of magnetic fields from loop 1504 to loop 1506. In oneembodiment, loop 1506 has a diameter of approximately 150 mm (i.e.approximately six inches), to provide plasma processing of workpieceshaving diameters of 200 mm and 300 mm. Loops 1504 and 1506 are coaxialand thus include a common axis which intersects the center of workpiece54 when the workpiece is properly positioned on chuck 52. Because loop1506 is circular and essentially closed on itself, loop 1506approximates a perfect coil to provide plasma 50 with magnetic fieldsthat are close to being azimuthally symmetrical.

Reference is now made to FIG. 16 of the drawing, a schematic perspectiveview of the antenna including transformer 257 that is schematicallyillustrated in FIG. 12. The combined inductance of secondary windings346 and 348, as illustrated in FIG. 16, is substantially larger than theinductance of secondary winding 1006, as illustrated in FIG. 15. Thelarger combined inductance of secondary windings 346 and 348 facilitatesresonating the impedance associated with the coil of FIG. 16 to thefrequency of source 232 or 233.

Primary winding 258, constructed basically the same as primary winding1004, includes planar non-magnetic, metal loop 324 having ends 276 and278 that are coincident with terminals 214 and 220 and ohmicallyconnected to current feeds 213 and 222.

Secondary windings 346 and 348, together, include four substantiallycircular, nonmagnetic metal planar loops (i.e. turns) 341-344, eachformed as a sector of a circle, wherein each sector has an extent ofapproximately 340 degrees. Turns 341 and 342 of winding 346 are coplanarin a first plane while turns 343 and 344 of winding 348 are coplanar ina second plane. The first and second planes are parallel to each otherand the upper face of window 46. The planes including windings 346 and348 are also parallel to the plane of planar loop 324 of the primarywinding 258 of transformer 257. Each of loops 324, 341 and 343 has thesame diameter that is somewhat less than the equal diameters of loops342 and 344. All of loops 258 and 341-344 are coaxial with verticallyextending axis 350 that intersects the center of workpiece 54 when theworkpiece is correctly positioned on chuck 52.

The planes including windings 346 and 348 are relatively close to theupper surface of window 46, with plane 346 being farther from the windowthan plane 348. The planes including windings 346 and 348 areconsiderably closer to window 46 than the plane of loop 324, so thatsubstantial magnetic flux from loops 341-344 is coupled to plasma 50 anda relatively small amount of magnetic flux from loop 324 is directlycoupled to plasma 50. The planes including windings 346 and 348,however, are sufficiently close to the plane of loop 324 so substantialmagnetic flux is coupled from loop 324 to loops 341-344, to provide atransformer coupling coefficient between the primary winding includingloop 324 and the secondary winding including loops 341-344 in the rangeof 0.1 to 0.3, approximately.

Loops 341-344 are connected in series with each other so that the RFcurrent induced in them in response to magnetic flux resulting from RFcurrent flowing in primary winding 258 flows in the same direction in avertical half plane originating at axis 350 and extending through loops341-344. Thus, at one instant of time, the currents in the right handportions of loops 341-344 (as illustrated in FIG. 16) flow in ahorizontal plane toward a viewer. At the same instant of time, thecurrents in the left hand portions of turns 341-344 flow in a horizontalplane away from a viewer. Consequently, the magnetic fluxes originatingin turns 341-344 add in plasma 50.

To these ends, loop 341 includes a first end 352 connected by verticallyextending lead 354 to a first end 356 of loop 343, having a second end358 connected by lead 360 to a first end 362 of loop 344. Lead 360 iscoplanar with loops 343 and 344. Turn 344 has a second end 364 connectedby vertically extending lead 366 to a first end 368 of loop 342. Turn342 has a second end 370 connected by leads 371-373 to a second end 376of loop 341. Leads 371 and 373 extend vertically through the planeincluding loop 324 and have ends above the plane of loop 324. The upperends of leads 371 and 373 are connected together by horizontallyextending lead 372. Positioning leads 371-373 in this way decreases thetendency for current flowing in these leads affecting the magneticfluxes that loops 341-344 supply to plasma 50. Loops 341-344 areconnected in series with each other and capacitors 254 and 256 by leads354, 356, 366 and 371-373 in such a manner that loops 341-344 are closedon each other. The entire series circuit forms a closed loop in theantenna of FIG. 16. The fact that turns 341-344 are closed on each otherand are substantially circular assists in providing the desiredsubstantially symmetrical azimuthal magnetic flux coupling to plasma 50and substantially symmetrical azimuthal density of plasma 50.

Loops 342 and 343 include gaps, such that the gap in loop 342 is definedby terminals 268 and 270, while the gap in loop 343 is defined byterminals 272 and 274. Terminal 270 is substantially coincident with end368 of loop 342, while terminal 274 is substantially coincident with end356 of loop 343.

Capacitors 254 and 256 are respectively in the gaps of loops 342 and343. This location of capacitors 254 and 256 is such that theinductances of the turns between opposite electrodes of the capacitorsare approximately equal, resulting in a preferred distribution of thecurrent and voltage along the length of the closed loop includingsecondary windings 346 and 348. The values of capacitors 254 and 256,and the inductances of loops 258 and 341-344 and the mutual inductancebetween loop 258 and loops 341-344, result in the reactive impedance ofclosed loop 1202 including secondary windings 346 and 348 having aresonant frequency equal to a frequency in the range of frequencies thatsource 233 can derive. Microprocessor 231 controls the frequency ofsource 233 so that the impedance of source 233 is matched to theimpedance of the load the source drives, resulting in the frequency ofsource 231 nearly being equal to the resonant frequency of closed loop1202.

While there have been described and illustrated specific embodiments ofthe invention, it will be clear that variations in the details of theembodiments specifically illustrated and described may be made withoutdeparting from the true spirit and scope of the invention as defined inthe appended claims. For example, the matching network can have aconfiguration other than the specifically described configurations ofFIGS. 3-12; for example, an “L” or 111 configuration can be employed. Tosimplify the analysis, transmission line effects of the antenna, whichcan be significant at 13.56 MHz, have been ignored.

1. An antenna for a plasma processor, the antenna being adapted to bedriven by power from an AC source for exciting a plasma, comprisingfirst and second excitation terminals, a coil including (a) a primarywinding having opposite ends respectively coupled with the first andsecond excitation terminals, and (b) a secondary winding reactivelycoupled with the primary winding; and a capacitor having first andsecond opposite electrodes respectively connected in series with thesecondary winding.
 2. The antenna of claim 2 wherein the secondarywinding is arranged to be excited by the AC source only via the reactivecoupling with the primary winding.
 3. The antenna of claim 2 wherein acoupling coefficient between the windings is such that there is loosetransformer coupling of the impedance of the secondary winding to theprimary winding.
 4. The antenna of claim 3 wherein the couplingcoefficient is in the range of about 0.1 to 0.3.
 5. The antenna of claim4 wherein the secondary winding includes plural turns.
 6. The antenna ofclaim 5 wherein the turns are in different parallel planes adapted to bespatially parallel to a coupling window of a vacuum chamber of theprocessor.
 7. The antenna of claim 6 wherein the plural turns areconcentric with an axis of the coil, the primary winding including atleast one further turn that is concentric with the coil axis and is in aplane spatially parallel to the plural turns of the secondary winding.8. The antenna of claim 7 wherein the secondary winding includesmultiple turns in each of the planes.
 9. The antenna of claim 8 whereinthe turns of the secondary winding are connected in series with eachother and arranged so AC current induced to flow therein in response toexcitation of the primary winding flows in the same direction in halfplanes extending from the axis through the turns.
 10. The antenna ofclaim 9 wherein the turns of the secondary winding are closed onthemselves.
 11. The antenna of claim 1 wherein the secondary windingincludes plural turns that are closed on themselves.
 12. A plasmaprocessor including the antenna of claim 1, the processor including theAC source and a vacuum chamber having an interior arranged to beresponsive to electromagnetic fields derived by the antenna in responseexcitation by the AC source.
 13. An antenna for a plasma processor, theantenna being adapted to be driven by power from an AC source forexciting a plasma, comprising first and second excitation terminals, acoil including (a) a primary winding having opposite ends respectivelycoupled with the first and second excitation terminals, and (b) asecondary winding reactively coupled with the primary winding, thesecondary winding including plural turns.
 14. The antenna of claim 13wherein the secondary winding is arranged to be excited by the AC sourceonly via the reactive coupling with the primary winding.
 15. The antennaof claim 14 wherein a transformer coupling coefficient between thewindings is such that there is loose coupling of the impedance of thesecondary winding to the primary winding.
 16. The antenna of claim 15wherein the coupling coefficient is in the range of about 0.1 to 0.3.17. The antenna of claim 14 wherein the turns are in different parallelplanes adapted to be spatially parallel to a coupling window of a vacuumchamber of the processor.
 18. The antenna of claim 17 wherein the pluralturns are concentric with an axis of the coil, the primary windingincluding at least one further turn that is concentric with the coilaxis and is in a plane spatially parallel to the plural turns of thesecondary winding.
 19. The antenna of claim 18 wherein the secondarywinding includes multiple turns in each of the planes.
 20. The antennaof claim 19 wherein the turns of the secondary winding are connected inseries with each other and arranged so AC current induced to flowtherein in response to excitation of the primary winding flows in thesame direction in half planes extending from the axis through the turns.21. The antenna of claim 20 wherein the turns of the secondary windingare closed on themselves.
 22. The antenna of claim 21 wherein the pluralturns are closed on themselves.
 23. The antenna of claim 22 wherein atransformer coupling coefficient between the windings is such that thereis loose coupling of the impedance of the secondary winding to theprimary winding.
 24. A plasma processor including the antenna of claim23, the processor including the AC source and a vacuum chamber having aninterior arranged to be responsive to electromagnetic fields derived bythe antenna in response to excitation by the AC source.
 25. A plasmaprocessor including the antenna of claim 14, the processor including theAC source and a vacuum chamber having an interior arranged to beresponsive to electromagnetic fields derived by the antenna in responseto excitation by the AC source.